This invention relates to radioactive well logging and more particularly to pulsed neutron logging.
Perhaps the most widely used of the radioactive logging procedures are the neutron logging techniques in which a formation under investigation is irradiated with neutrons and the resulting secondary radiation measured in order to characterize the formation. The neutron irradiation may be by means of a continuous source or a pulsed source, and the secondary radiation detected typically will take the form of thermal or epithermal neutrons or gamma rays such as may result from inelastic scattering reactions or neutron capture. In pulsed neutron logging, the formation is bombarded with repetitive time-spaced bursts of fast neutrons, and the resulting secondary radiation is measured at selected time intervals, normally by gating the output of the detector, in order to arrive at a decay parameter.
A pulsed neutron generator for such a system commonly takes the form of a three-element, linear accelerator tube. It includes a replenisher element which is electrically heated to boil off deuterium gas absorbed by the filament. The deuterium molecules are ionized by an ionizing section which commonly includes plates to which a positive ionization pulse is applied. The deuterium ions are then accelerated and bombard a target which includes tritium molecules. The bombardment of the deuterium ions on the tritium molecules yields helium plus a supply of neutrons. One commercially available tube which is capable of such operation is the Kaman Nuclear Model A-801 Neutron Generator.
Such accelerator type neutron sources have a high-energy, high-intensity monoenergetic neutron output which varies widely and unpredictably in intensity. Hence, it is desirable to monitor the output to know that a constant output is produced during each measuring period or to correct or compensate for variations in the neutron output. The response time of a conventional fast neutron detector is not fast enough to detect directly and measure accurately the number of neutrons produced by the source when it is being operated to produce neutron bursts having a time duration of a few microseconds or less.